Secret Universe: The Hidden Life of the Cell 

The smallest structural unit of an organism which is capable of independent functioning, (i.e., it has an independent metabolism and the capacity to reproduce (with a few exceptions within the human body)); it consists of one or more nuclei, cytoplasm, and various internal organelles, all surrounded by a semipermeable plasma cell membrane; it is capable, either independently and alone, or interacting with other related units of performing all the fundamental functions of life; "the unit of life."   [All cells have their origin in the primary cell from which the organism was developed.]